Olefinic hydrocarbons are feedstocks for a variety of commercially important reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the position of the double bond or the degree of branching of the hydrocarbon, may be important to the efficiency of the chemical reaction or the properties of the product. The distribution of isomers in a mixture of olefinic hydrocarbons is rarely optimum for a specific application. It is often desirable to isomerize olefins to increase the output of the desired isomer.
Butenes are among the most useful of the olefinic hydrocarbons having more than one isomer. A high-octane gasoline component is produced from a mixture of butenes in many petroleum refineries, principally by alkylation with isobutene; 2-butenes (cis- and trans) generally are the most desirable isomers for this application. Secondary butyl alcohol and methylethyl ketone, as well as butadiene, are other important derivatives of 2-butenes. Demand for 1-butene has been growing rapidly, based on its use as a co-monomer for linear low density polyethylene and as a monomer in polybutene production. Isobutene finds application in such products as methyl methacrylate, polyisobutene and butyl rubber. The most important derivative influencing isobutene demand and butene isomer requirements, however, is methyl tertiary butyl ether (MTBE) which is experiencing rapid growth in demand as a gasoline component.
Pentenes are also valuable olefinic feedstocks for fuel and chemical products. Isoprene, which may be produced by the dehydrogenation of isopentene or by the extraction of steam cracker C.sub.5 hydrocarbon product, is an important monomer in the production of elastomers. To an increasing extent, pentenes obtained from refinery cracking units are alkylated with isobutane to obtain a high octane gasoline component. The principal influence on trends in isopentene demand and pentene isomer requirements, however, is the rapid growth and demand for tertiary amyl methyl ether (TAME) as a gasoline component. TAME is of increasing interest as restrictions on gasoline olefins and volatility reduce the utility of pentenes as a gasoline component. This interest may extend to hexenes and higher olefins having tertiary carbons which could be reacted to yield high octane ethers.
Only rarely are olefin isomers obtained in a refinery or petrochemical product in a ratio-matching product demand. In particular, there is a widespread need to increase the proportion of isobutene, isopentene and other tertiary-carbon olefins for the production of MTBE, TAME, ethyl tertiary butyl ether (ETBE), and tertiary amyl ethyl ether (TAEE).
Processes for the production of such ethers have suffered from a shortage of the isoolefins necessary for reaction with the alcohols to provide the desired ethers. Feed streams for etherification processes typically consist of a wide variety of olefinic and paraffinic isomers. The availability Of etherification feedstocks have been increased through the dehydrogenation of paraffins and through the skeletal isomerization of olefins.
The skeletal isomerization of olefins involves the reorientation of the molecular structure in respect to the formation or elimination of side chains. More particularly, skeletal isomerization relates to the conversion of unbranched olefins into branched olefins having the same number of carbon atoms. The skeletal isomerization of olefins is known to be accomplished by contacting unbranched or slightly branched olefins with an acidic catalyst at elevated temperatures. The process is generally applicable to olefins having about 4 to about 20 carbon atoms per molecule and is especially applicable to olefins having about 4 to about 10 carbon atoms per molecule. The process may be used to form isobutene from normal butenes.
U.S. Pat. No. 4,554,386 (issued to Groeneveld) discloses a combination etherification and skeletal isomerization process for making MTBE. In this process, a first MTBE reactor is supplied with an isobutene-containing hydrocarbon stream and a methanol stream. These streams are then reacted in the presence of an etherification catalyst. The effluent from the MTBE reactor is sent to a first MTBE separation column. From this column, MTBE is discharged from the bottom and a stream containing unconverted isobutene, methanol, side products (e.g. dimethyl ether), normal butenes, and butanes are discharged as overhead and recycled to the isomerization reactor. The effluent from the isomerization reactor is sent to a second MTBE reactor to complete the etherification/alkene isomerization loop. To avoid the buildup of alkanes in the system, lower molecular weight hydrocarbons are purged using a fractionation column.
The problem with discharging or purging light ends from an olefin isomerization process by fractionation is that the light ends contain a significant amount of valuable alkenes which can be lost along with the undesirable alkanes because the boiling points of the alkenes are very close to the boiling points of the alkanes, i.e., the boiling points of the isoalkenes used for ether production are between the boiling points of the alkanes that are to be purged. As a result of the loss of these alkenes, the overall ether yield is reduced.
There is a need for an alkane isomerization process that avoids the buildup of alkanes in the process without the loss of valuable alkenes.